The good news is that's totally normal. "It doesn’t mean you’re out of shape," says Jordan Syatt, a certified personal trainer and Greatist expert. "You just elevated your heart rate and need more oxygen." Unless you’re doing staircase workouts or using the StairMaster religiously, a quick burst of energy can leave you short of breath, says Nieca Goldberg, M.D., a cardiologist and director of the Joan H. Tisch Center for Women’s Health at NYU Langone Medical Center.

Climbing a flight of stairs uses more muscles than the simple act of walking, Syatt says. After all, you're essentially doing lunges uphill (and fighting gravity in the process). And a move like that might be slightly more advanced than most people's fitness level, Goldberg says. As any trainer will tell you, once a workout becomes too easy, you need to add intensity if you want to keep seeing progress. At the new level of difficulty, you’ll be challenged again.

Another potential culprit? If you’re already working out vigorously to train for a strenuous event, like a half or full marathon, getting up a flight of stairs is just contributing to your already heavy workload. Running up 20 stairs, as opposed to running 20 paces on flat land, combines an aerobic activity with a strength activity, says Jason Fitzgerald, a certified track and field coach and Greatist expert. “Even if you’re in great shape, that’s going to get you out of breath very quickly,” Fitzgerald says.

Your Action Plan

"Like anything, you’ll get better at it with practice," Fitzgerald says. Syatt suggests incorporating lower-body strength exercises like split squats, lunges, and reverse lunges into your workouts to help mimic the movement of going up stairs.

"Start with your own body weight, then eventually add 15, 30, or even 40 pounds," Syatt says. "When you're walking up the stairs, it'll be a lot easier because you're more conditioned for it."

So go ahead and breathe a sigh of relief—as soon as you get to the top, that is.